Scientists quietly announce a potentially huge discovery in physics

Lost amid the tragedy of Monday’s explosions in Boston was a fascinating piece of physics news: an unexpected glimpse of what is very likely dark matter.

Sam Ting got all the headlines two weeks ago with his first release of data from the Alpha Magnetic Spectrometer, but this news from an Earth-bound detector may ultimately prove more momentous.

Here’s what happened, and why it’s so interesting.

WIMPs

A leading candidate for dark matter, which scientists believe makes up about 27 percent of the universe but which they have never directly observed, are the delightfully named WIMPs, or weakly interacting massive particles. These are theoretical, and they are just what they’re called: they almost never interact with normal matter, and they’re large because they make up a lot of the universe.

Dark matter is out there. Can scientists find it?

But “almost never” interacting with normal matter doesn’t mean never. And scientists believed with the right kind of detector they could observe WIMPs.

Between 2006 and 2008 about four dozen physicists buried 19 Germanium-based detectors and 11 silicon-based detectors deep in a mine in Minnesota. They believed the Germanium detectors might be just right to capture the rare, but theoretically possible collision between a WIMP and an atomic nucleus. The silicon detectors were just there to confirm the result — i.e. if a Germanium detector recorded such a collision and a silicon detector did not, that would be good evidence for a WIMP.

Why? Because theoretical physicists guessed that WIMPs would be about 50 times the size of a proton. Because mass and energy are equivalent thanks to Einstein’s E=mc2, particle physicists measure mass in energy units. A proton has a mass of 1 billion electron volts, or 1 GeV. Physicists thought WIMPs might have a mass of about 50 GeV.

Where are the big WIMPs?

After taking their data for three years the scientists got a ho-hum result — the Germanium detectors recorded two events, when on average they would have expected to see 0.9 events during the time period. This was not statistically significant, and moreover, they later concluded these events were attributable to the leakage of electrons.

Since the primary detectors showed no significant results, data collected by the silicon detectors, which could only detect WIMPs up to a mass of about 15 GeV were not analyzed.

Then, after some considerations, the physicists came to believe that maybe the WIMPs weren’t really, really big. So they went back and studied the silicon detector data and found three events, when they would have expected just 0.7 events during the time period of data taking. This is statistically significant.

They then spent three months trying to find other explanations for the data, and did not succeed. In absence of other discoveries, the most likely outcome here is that, for the first time ever, scientists have directly detected long-sought and mysterious dark matter.

WIMPs found?

So they published their results on Monday (see paper). Based upon their statistical analysis, they are 99.8 percent sure they have observed some WIMPs at a mass of about 8 GeV. But in particle physics, certainty doesn’t come until they are 99.9999 percent sure.

Mahapatra. (Texas A&M)

“In our field you can easily be wrong, and 99.8 percent sure is not enough,” said Rupak Mahapatra, a Texas A&M University physicist involved with the experiment. “We need more data, bigger detectors and other experiments that could confirm our finding.”

Such experiments are coming. Mahapatra said the technology to build larger silicon detectors already exists, and there are tentative plans to install them in Snolab in Canada. But physics is often a waiting game, and data might not be available until 2020.

“One must have time, patience and money,” Mahapatra said.

Implications of smaller WIMPs?

So what do theoretical physicists think about the possibility of smaller WIMPs? The short answer is: intriguing.

Bhaskar Dutta, a theoretical physicist at Texas A&M University, believes there may be a tantalizing clue about the nature of the universe in the mass of the WIMPs. A proton, he notes, has a mass of 1 GeV. Therefore the ratio of mass between a proton and the WIMPs likely found by the experiment is 1-to-8.

Thus the ratio of baryons — that is normal matter including protons — to dark matter in the universe could be about 1-to-6.

“This may be a clue to the fundamental origin of matter and antimatter,” Dutta says. In other words, it may not be a coincidence that the number of protons and neutrons in the universe is roughly equivalent to the number of WIMPs.

If such a ratio is valid it would allow physicists to make predictions that other instruments, such as the Large Hadron Collider in Switzerland, could test.

The bottom line is that while there’s still a lot of uncertainty about the nature of dark matter, it feels like physicists are closing in on the answer.